Asymmetric Cu(I)─W Dual‐Atomic Sites Enable C─C Coupling for Selective Photocatalytic CO2 Reduction to C2H4

Abstract Solar‐driven CO2 reduction into value‐added C2+ chemical fuels, such as C2H4, is promising in meeting the carbon‐neutral future, yet the performance is usually hindered by the high energy barrier of the C─C coupling process. Here, an efficient and stabilized Cu(I) single atoms‐modified W18O49 nanowires (Cu1/W18O49) photocatalyst with asymmetric Cu─W dual sites is reported for selective photocatalytic CO2 reduction to C2H4. The interconversion between W(V) and W(VI) in W18O49 ensures the stability of Cu(I) during the photocatalytic process. Under light irradiation, the optimal Cu1/W18O49 (3.6‐Cu1/W18O49) catalyst exhibits concurrent high activity and selectivity toward C2H4 production, reaching a corresponding yield rate of 4.9 µmol g−1 h−1 and selectivity as high as 72.8%, respectively. Combined in situ spectroscopies and computational calculations reveal that Cu(I) single atoms stabilize the *CO intermediate, and the asymmetric Cu─W dual sites effectively reduce the energy barrier for the C─C coupling of two neighboring CO intermediates, enabling the highly selective C2H4 generation from CO2 photoreduction. This work demonstrates leveraging stabilized atomically‐dispersed Cu(I) in asymmetric dual‐sites for selective CO2‐to‐C2H4 conversion and can provide new insight into photocatalytic CO2 reduction to other targeted C2+ products through rational construction of active sites for C─C coupling.


Characterizations
The morphology and microstructure of the samples were characterized by SEM (Hitachi S-4800) and HRTEM (JEOL JEM-2100 F).HAADF-STEM images and elemental mapping were collected at 300 kV on a ThermoFisher Scientific Spectra 300 scanning transmission electron microscope equipped with a Super-X EDS detector system.The phase and structural information of the samples were detected by XRD (Bruker AXS D8 diffractometer).To ascertain the chemical composition and valence state, the samples were studied by XPS (Thermo Fisher Scientific Escalab 250 spectrometer).UV-vis absorption spectra were recorded by a Shimadzu UV 2550 spectrophotometer.The content of Cu in the x-Cu1/W18O49 samples was measured by ICP-MS on a PerkinElmer NexION 1000G instrument.The oxygen vacancies were recorded by EPR (Bruker EMXplus).CO temperature-programmed desorption (CO-TPD) is tested on AutoChem1 II 2920.

XAFS Measurements and Analysis
XAFS analyses were performed with Si (111) crystal monochromators at the BL14W beamline at the Beijing Synchrotron Radiation Facility.Before the analysis at the beamline, samples were placed into aluminum sample holders and sealed using Kapton tape film.Particular care was taken to minimize the beam-induced oxidation of the samples by placing the sample stands filled with pristine and reacted samples in a nitrogen-filled glove box for 6 hours before transferring them to zippered bags in this glove box.The XAFS spectra were recorded at room temperature using a 4-channel Silicon Drift Detector (SDD) Bruker 5040.The EXAFS spectra were recorded in fluorescence mode.The acquired EXAFS data were processed according to the standard procedures using the Athena program implemented in the IFEFFIT software packages.The normalized EXAFS spectra were obtained by setting the pre-edge and post-edge to 0 and 1, respectively.Then, the χ (k) data were Fourier transformed into real (R) space by a Hanning window with sill size of 1.0 Å −1 to separate the EXAFS contributions from the distinct coordination shells.To obtain quantitative structural parameters around Cu atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of the IFEFFIT software packages.

Photocatalytic CO2 Reduction
For the photocatalytic CO2 reduction reaction, 5 mg of catalyst was dispersed in 500 μL of aqueous solution and then dropped onto quartz fiber paper (diameter 25 mm).
Then it was dried in a vacuum oven at 60°C.The quartz fiber paper containing the catalyst was placed at the bottom of the reactor and 200 μL of water was added dropwise to the reactor.This was followed by the introduction of CO2 feeding gas for 30 min to expel the air.A 300W Xe lamp (Beijing China Education Au-light Co.) with a light intensity of 453 mW cm −2 was used as the light source.After the reaction, the generated products were quantified by a FuLi GC9790 gas chromatography (GC).
The selectivity of C2H4 is calculated as follows: Selectivity of where   2  4 ,   4 and   represent the yield of C2H4, CH4 and CO during the photocatalytic reactions, respectively.

C Isotope Labeling Experiments
In the 13 C isotope labeling experiment, 5 mg of catalyst was dispersed in 500 μL of aqueous solution and then dropped onto quartz fiber paper (diameter 25 mm).Then it was dried in a vacuum oven at 60 °C.The quartz fiber paper containing the catalyst was placed at the bottom of the reactor and 200 μL of water was added dropwise to the reactor.This was followed by the introduction of 13 CO2 feeding gas into a quartz reactor (50 mL) previously vented by argon.A 300W Xe lamp was used as the light source, and the final products were detected by Shimazu GCMS-QP2020.

In-situ NAP-XPS Measurements
In-situ NAP-XPS measurements were performed at the photoemission end-station at beamline BL11U in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China.The end station is composed of three chambers-an analysis chamber, a preparation chamber, a load-lock chamber.The catalyst was dropcast onto a clear silicon wafer, and dried at room temperature.High-purity CO2 (99.9999%) was introduced into the analysis chamber with the partial pressure maintained at 0.25 mbar.
Then, the sample was transferred to the analysis chamber for XPS measurement without exposing it to air.A Xe lamp source (PLS-SXE300D, Beijing Perfectlight) was used as a light source.

In-situ DRIFTS Measurements
In-situ DRIFTS measurements were performed using a Thermo Scientific Nicolet iS50 FT-IR with a liquid-nitrogen-cooled HgCdTe detector.Each spectrum was recorded by averaging 64 scans at a resolution of 4 cm −1 .The sample was held in a customfabricated IR reaction chamber which was specifically designed to examine highly scattered powder samples in diffuse reflection mode.The chamber was sealed with two ZnSe windows.The spectra were collected under dark conditions or after a certain irradiation time using a 300 W Xe lamp (PerfectLight, China).The spectra were obtained by subtracting the background from the spectra of samples.

DFT Calculations
DFT calculations were performed using the quantum espresso (QE) based on the pseudopotential plane wave (PPW) method.The Perdew-Bueke-Ernzerhof (PBE) functional was used to describe exchange-correlation effects of electrons.We have chosen the projected augmented wave (PAW) potentials to describe the ionic cores and take valence electrons into account using a plane wave basis set with a energy cutoff of 400 eV in all relaxation processes.The convergence criteria were set to 10 −5 eV for the energy and −0.001 eV/Å for the force.The k-point meshes were set of 2 × 3 × 1, 3 × 3 × 1, 7 × 7 × 1 for geometry optimization, electronic self-consistent, and the electronic structure calculation, respectively.All constructions possess larger than 15 Å vacuum region to minimize the interactions between adjacent image cells.
According to the computational hydrogen electrode (CHE) model proposed by Nørskov.Herein, the free energy (H + + e − ) of the electron-proton pair can be regarded as the chemical potential of 1/2 gaseous H2 in equilibrium (0 V for standard hydrogen electrode).The Gibbs free energy between each electrocatalytic reaction step was obtained by the following equation: Where ∆E, ∆EZPE and ∆S represent the difference of adsorption energy, zero-point energy, and entropy at 298.15 K of the reaction intermediate on the substrate, respectively.The zero-point energies and entropies of the reaction intermediate were calculated through vibration frequencies, in which we fix the substrate and allow the absorb rate vibrational modes to be computed.Table S3.The comparison of C2H4 yield rate and selectivity for the 3.6-Cu1/W18O49 photocatalyst with those recently reported Cu(I)-based photocatalysts for CO2 reduction to C2+ products.
Table S4.The comparison of C2H4 yield rate and selectivity for the 3.6-Cu1/W18O49 photocatalyst with those recently reported systems for photocatalytic CO2 reduction to C2+ products.
Table S5.Free energy of CO adsorption at different sites.

Figure S9 .
Figure S9.Image of the reaction device for photocatalytic CO2 reduction.

Figure S28 .
Figure S28.The side views of atomic structures of reaction intermediates on W18O49 for *CO coupling.Gray ball: W, red ball: O, brown ball: C and cyan ball: H.